1. Field of the Invention
The present invention relates to a safety valve for the venting circuit of a liquid tank, in particular a fuel tank with which a motor vehicle may be equipped.
Liquid tanks, in particular fuel tanks for motor vehicles, are nowadays generally provided inter alia with a venting circuit. This circuit allows air to be introduced into the tank in the event of underpressure (especially for compensating for the volume of liquid consumed) or allows the gases contained in the tank to be removed in the event of overpressure (especially in the event of overheating). This circuit also allows the ducting and possible filtering of the gases that have to be discharged into the atmosphere, for the purpose of meeting the ever stricter environmental requirements in this regard.
2. Description of the Prior Art
The venting circuit generally includes a safety valve of the ROV (roll-over valve) type which as far as possible prevents liquid from coming out of the tank in the event of said tank rolling over or being at an excessively high angle of inclination. This safety valve must also respond rapidly and reliably when its intervention conditions occur, but with minimum sensitivity to abnormal phenomena such as especially a very high flow rate, overpressure in the tank or low-amplitude waves.
The venting circuit may also include a safety valve of the FLVV (fill limit venting valve) which sets the maximum fill level of the tank and limits overfilling.
Some safety valves provide both the ROV function and the FLVV function.
Many safety valves employ a float, comprising an upper needle that closes off an aperture for connection between the tank and the venting circuit. The disadvantage of such valves is that when the aperture is closed off for a long time (for example when the tank remains for a long period at its maximum fill level), the pressure rises inside the tank and can no longer be relieved by said aperture, hence the occurrence of an overpressure that has to be relieved by an additional device.
To alleviate this drawback, valves with a moveable valve seat have been developed. Thus, for example, patent U.S. Pat. No. 5,004,002 discloses a valve with a needle float and a moveable valve seat. This valve seat may be a diaphragm pierced by an aperture (see FIG. 5), which rises above a certain pressure so as to separate the needle of the float from said aperture. The valve described in that patent was developed for fuel systems that have to withstand a relatively high pressure level. Consequently, to prevent the needle sticking in the aperture, the movement of the float is blocked by means of a stop so that only the diaphragm can move above a certain pressure. The drawback of such a valve is therefore that, if a wave occurs when the diaphragm and the needle are decoupled (that is to say during relief of an overpressure), liquid can be entrained into the system, (since the float is blocked and cannot move in order to close off the aperture). Furthermore, in this very situation and/or in the event of a large overpressure, there is a risk of the central part of the diaphragm remaining stuck against the upper wall of the valve, and thus blocking the venting aperture.